1. Field of the Invention
The present invention relates to correlators. More specifically, the present invention relates to range correlators.
While the present invention is described herein with reference to illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those having ordinary skill in the art and access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the present invention would be of significant utility.
2. Description of the Related Art
Range correlators generally provide an indication of range by timing the roundtrip delay of a transmitted pulse. In order to identify the return pulse, the returns are correlated with a known transmitted pulse. Many techniques are known in the art for performing the correlation. These techniques generally involve the digitizing of the return and the bit-by-bit matching thereof with a digitized representation of the transmitted pulse. The range bin with the best correlation provides the desired indication of range.
Time compression has been recognized as an advantageous correlation technique which is well adapted for range correlation. Time compression involves the transmission of a pulse, phase modulated in accordance with a correlation code, e.g., a Barker code. The return pulse is demodulated with the correlation code and the bits of each word of the resulting code are accumulated for each of a number of time slots, each time slot corresponding to a particular range bin. The time slot with the highest accumulated total is identified as corresponding to the range bin of the target.
Charge coupled devices (CCDs) are frequently used for time compression correlation. In a typical CCD correlator, analog samples of the return pulse are shifted one sample at a time through a register. The register is N bits long where N=code length. The signal returns either in phase or 180 degree out of phase for each PN code chip width. The N bits are all summed together, the 180 degree out of phase samples are inverted back to n-phase prior to summation. Since each register in the CCD corresponds to a time slot and hence a range bin, the register with the highest sum is associated with the target range bin.
CCDs provide fast sample and hold, and slow storage for correlation. Nonetheless, CCDs have been found to be somewhat problematic when used in correlators. The manufacture of CCDs is not easily repeatable. That is, CCDs are difficult to manufacture consistently. The operating characteristics of the devices are known to vary unpredictably and nonuniformly with temperature, voltage, frequency, code length, offsets, etc. from device to device. This makes it difficult to keep the devices within specification and exacerbates the critical matching of the devices required for optimum performance in a correlator. The net result is a high cost associated with the manufacture, implementation and operation of a CCD correlator.
Thus, there is a need in the art for a correlation system capable of performing time compression without the aforementioned shortcomings of CCD correlators.